The present invention generally relates to the art of optical telecommunications and more particularly to an optical telecommunication apparatus for use in a node forming a part of an optical ring network.
FIG. 1 shows a typical example of a two-fiber bidirectional-line-switched-ring (2F-BLSR) network 10 that is used commonly in current optical telecommunication networks.
Referring to FIG. 1, the 2F-BLSR network 10 includes nodes 11.sub.1 -11.sub.4 connected with each other by an optical fiber link, wherein the nodes 11.sub.1 -11.sub.4 are connected each other successively in a counterclockwise direction by a first optical link called a work link Wk and a second, redundant optical link called a protect link P, wherein the protect link connects the nodes 11.sub.1 -11.sub.4 in a clockwise direction. Thus, the work link Wk carries optical signals traveling in the counter-clockwise direction in the ring 10, while the protect link P carries optical signals traveling in the ring 10 in the clockwise direction.
Thus, when there occurs a failure in the work link Wk, the ring network continues functioning properly by using the protect link P. For this purpose, each of the nodes 11.sub.1 -11.sub.4 includes a switch circuit as will be explained later.
FIG. 2 shows a typical example of a four-fiber bidirectional-line-switched-ring (4F-BLSR) network 20 that is proposed for enhancing the capability of the network to maintain a connection upon occurrence of a severe failure in the link.
Referring to FIG. 2, the 4F-BLSR network 20 includes nodes 21.sub.1 -21.sub.4 connected with each other by an optical fiber link, wherein the nodes 21.sub.1 -21.sub.4 are connected in the counter-clockwise direction by a work link Wk1 and a protect link P2 and further in the clockwise direction by a work link Wk2 and a protect link P1.
Thus, in the 4F-BLSR network 20, any failure occurred in the second work link Wk2 is immediately saved by using the protect link P1 as indicated in FIG. 3A. Similarly, any failure occurred in the work link Wk1 is immediately saved by using the protect link P2. Further, the 4F-BLSR network 20 is capable of saving the failure occurred in all of the links connecting a pair of mutually adjacent nodes such as the node 21.sub.1 and the node 21.sub.4, by forming a link between the nodes 21.sub.1 and 21.sub.4 by the first and second protect links P1 and P2 over the nodes 21.sub.2 -21.sub.4 as indicated in FIG. 3B.
FIG. 4 shows the construction of the node 11.sub.1 used in the 2F-BLSR system of FIG. 2 in the form of a block diagram. As other nodes 11.sub.2 -11.sub.4 have substantially the same construction, the description thereof will be omitted.
Referring to FIG. 4, the node 11.sub.1 includes a reception unit (11A).sub.WK to which a high-speed multiplexed optical signal comes in from the node 11.sub.4 located at an East-side via the work link WK and another reception unit (11A).sub.P to which a multiplexed optical signal is supplied from the node 11.sub.2 located at a West-side via the protect link P. The reception unit (11A).sub.WK includes a photo reception device RC(1) for detection of the incoming optical signal and a demultiplexer device DM(1) for demultiplexing the multiplexed signal detected by the photoreception device RC(1) into individual signal components. Similarly, the reception unit (11A).sub.P includes a photo reception device RC(2) corresponding to the photo reception device RC(1) and a demultiplexer device DM(2) corresponding to the demultiplexer device DM(1).
It should be noted that each of the photo reception devices RC(1) and RC(2) receives an optical transmission having a bit-rate of 1.2 Gbps, wherein the bit-rate of 1.2 Gbps corresponds to a transmission of 24 channels each channel having a bit-rate of 50 Mbps. Thus, the photo reception device RC(1) receives the optical transmission for the first twenty-four channels (CH1-CH24) while the photo reception device RC(2) receives the optical transmission for the second twenty-four channels (CH25-CH48).
The signals for the channels CH1-CH24 thus detected by the photo reception device RC(1) are then demultiplexed by the demultiplexer device DM(1) in the photo reception device RC(1), and the signals for the first twenty-four channels (CH1-CH24) thus demultiplexed are supplied to a first switch unit (11B).sub.WK and further to a second switch unit (11B).sub.P. Similarly, the signals for the channels CH25-CH48 are demultiplexed by the demultiplexer device DM(2) and are supplied to the first switch unit (11B).sub.WK as well as to the second switch unit (11B).sub.P.
The first and second switch units (11B).sub.WK and (11B).sub.P, on the other hand, cooperate with each other and supply the signals of the first twenty-four channels (CH1-CH24) and the signals of the second twenty-four channels (CH25-CH48) selectively to one of first and second optical transmission units (11C).sub.WK and (11C).sub.P connected respectively to the work link Wk and the protect link P at a West-side of the node, wherein each of the optical transmission units (11C).sub.Wk and (11C).sub.P includes a multiplexer devices MX(1) or MX(2) and a laser transmitter device TC(1) or TC(2).
Thus, when there is a failure in the work link Wk at the West-side of the node 11.sub.1 leading to the node 11.sub.2, the switch unit (11B).sub.WK switches the path of the signals for the channels CH1-CH24, such that the signals of the channels CH1-CH24 are supplied from the photoreception unit (11A).sub.Wk at the East-side to the optical transmission unit (11C).sub.P also at the East-side. Thereby, the signals are transmitted from the node 11.sub.1 to the node 11.sub.2 via the protect link P over the nodes 11.sub.4 and 11.sub.3 consecutively. Simultaneously, the switch unit (11B).sub.P switches the signal path such that the photoreception unit (11A).sub.P and the optical transmission unit (11C).sub.P are disconnected.
Further, each of the nodes 11.sub.1 -11.sub.4 includes an add/drop unit A/D in the switch unit (11B).sub.Wk or (11B).sub.P as indicated in FIG. 4 for an add/drop control of optical signals in anticipation of connecting the node to an external optical network or link outside the 2F-BLSR network 10.
Conventionally, the apparatus of FIG. 4 has been used successfully in a node of an optical network in a state that the apparatus is accommodated in NEBS (Bellcore spec) standard open-rack frame structure typically having a height of 2100 mm (7 feet), a width of 660 mm and a depth of 305 mm (12 inches) as indicated in FIGS. 5A or 5B, wherein both of FIG. 5A and FIG. 5B show a frame 30 or 30' for a front-side loading of devices. In the frame 30 of FIG. 5A, it should be noted that the cables, provided along one or both of the side pillars, are connected to the corresponding devices on the frame 30 at a rear side thereof as indicated in FIG. 6A. In the frame 30' of FIG. 5B, on the other hand, the cables are connected to the corresponding devices at a front side thereof as indicated in FIG. 6B.
As the size of the frame is thus limited, it is necessary to reduce the size of the devices loaded on the frame as much as possible. The optical telecommunication apparatus of FIG. 4 is no exception.
As long as the optical telecommunication apparatus on the frame is used to handle the optical bit-stream of 2.4 Gbps, 1.2 Gbps for the work channels CH1-CH24 and 1.2 Gbps for the protect channels CH25-CH48, no problem occurs. The apparatus of FIG. 4 can be formed by merely connecting the various units by a limited number of coaxial cables. The apparatus thus assembled easily fits into the standard frame of FIGS. 5A or 5B.
When the bit-rate of the incoming optical signals is increased, however, there arises various difficulties as noted below.
FIG. 7 shows an example of the circuit in which the telecommunication circuit of FIG. 4 is expanded in a straightforward manner so as to handle the optical transmission of 9.6 Gbps in total, 4.8 Gbps of which are for the work channel and 4.8 Gbps of which are for the protect channel. In FIG. 7, those parts corresponding to the parts described already with reference to FIG. 4 are designated by the same reference numerals and the description thereof will be omitted.
Referring to FIG. 7, it should be noted that the incoming multiplexed optical signal on the work link Wk now includes 1-96 channels each carrying optical signals with a bit-rate of 50 Mbps. In correspondence to this, the circuit of FIG. 7 uses four switch devices MM1-MM4 for the switch unit (11B).sub.Wk each capable of handling signals of 24 channels or 1.2 Gbps bit-rate. In all, the switch unit (11B).sub.Wk of the apparatus of FIG. 7 is capable of handling 4.8 Gbps bit-rate signals. Thus, the switch device MM1 handles the channels 1-24 for the work channel or the channels 97-120 for the protect channel, the switch device MM2 handles the channels 25-48 for the work channel or the channels 121-144, the switch device MM3 handles the channels 49-72 for the work channel or the channels 145-168 for the protect channel, and the switch device MM4 handles the channels 73-96 for the work channel or the channels 169-192 for the protect channel.
In FIG. 7, it should be noted that there are another four switch devices MM5-MM8 for the switch unit (11B).sub.P of the protect link, wherein the switch devices MM5-MM8 have a similar construction to the switch devices MM1-MM4. Thus, the number of wirings for the circuit of FIG. 7 becomes, although not illustrated in FIG. 7, four times as large as that of the circuit of FIG. 4, and interconnection of various units by way of coaxial cables becomes difficult. This is particularly true when the overall size of the apparatus has to be adapted so as to fit into the standard frame of FIGS. 5A or 5B.
FIG. 8 shows an example of the telecommunication apparatus in which the apparatus of FIG. 4 is expanded in a straightforward manner so as to be capable of forming a 4F-BLSR network.
Referring to FIG. 8 showing the constitution of the node 21 of FIG. 4 as an example, the apparatus includes two series of circuits somewhat similar to the circuit of FIG. 7 respectively at the east side and the west side, such that the high-speed multiplexed optical signals on the work link Wk1 and the protect link P2 are received respectively by a photoreception unit (21A).sub.Wk1 and a photoreception unit (21A).sub.P2 at the east side of the node 21. Similarly, the high-speed optical signals on the work link Wk2 and the protect link P1 are received respectively by a photoreception unit (21A).sub.Wk2 and (21A).sub.P1 at the west side of the node 21. Similarly as before, each of the photoreception units includes a photoreception device designated as RC(1) or RC(2) and a demultiplexer device designated as DM(1) or DM(2).
Each of the photoreception units (21A).sub.Wk1, (21A).sub.Wk2, (21A).sub.P1 and (21A).sub.P2 supplies an output signal corresponding to the incoming high-speed optical signal to corresponding one of switch units (21B).sub.Wk1, (21B).sub.Wk2, (21B).sub.P1 and (21B).sub.P2, wherein each of the switch units (21B).sub.Wk1, (21B).sub.P1, (21B).sub.Wk2 and (21B).sub.P2 includes four switch circuits. The switch unit (21B).sub.Wk1 includes the switch devices MM1-MM4, while the switch unit (21B).sub.P2 includes the switch devices MM5-MM8, wherein each of the switch devices MM1-MM8 handles signals of 48 channels. For example, the switch device MM1 of the switch unit (21B).sub.Wk1 handles the signals of the channels 1 through 24 supplied from the photoreception unit (21A).sub.Wk1 as well as the signals of the channels 1-24 supplied from the photoreception unit (21A).sub.P2 at a first switch element SW and further the signals of the channels 1-24 supplied from the photoreception unit (21A).sub.Wk2 and the signals of the channels 1-24 supplied from the photoreception unit (21A).sub.P1 at a second switch element also designated by SW. Similarly, the switch device MM2 handles the channels 25-48 and the channels 25-48, the switch device MM3 handles the channels 49-72 and the channels 49-72, and the switch device MM4 handles the channels 73-96 and the channels 73-96. The other switch units (21B).sub.P2, (21B).sub.Wk2 and (21B).sub.P1 have a similar construction.
Further, each of the switch units (21B).sub.Wk1 -(21B).sub.P2 is connected to each of optical transmission units (21C).sub.Wk2 and (21C).sub.P1 at the west side as well as to each of optical transmission units (21C).sub.Wk1 and (21C).sub.P2 at the east side.
Thus, in the normal operational mode of the 4F-BLSR system shown in FIG. 2, the switch units (21B).sub.Wk1 -(21B).sub.P2 forward the optical signals incoming to the east side via the work link Wk1 and the protect link P2 of the east side respectively to the work link Wk1 and the protect link P2 of the west side and further the optical signals incoming to the west side via the work link Wk2 and the protect link P1 of the west side respectively to the work link Wk2 and the protect link P1 of the east side. When there occurs a failure in the link, the switch units (21B).sub.Wk1 -(21B).sub.P2 switches the path of the optical signals as indicated in FIGS. 3A or 3B. Thus, the work link Wk1 may become the work link Wk2 and the protective link P1 may become the protect link P2, or vice versa, depending upon the switching in the switch units (21B).sub.Wk1 -(21B).sub.P2.
It should be noted that the construction of FIG. 8 requires a very complex wiring between various units. Thus, it becomes extremely difficult or totally impossible to assemble the apparatus of FIG. 8 with a size such that the apparatus is accommodated in the standard open-rack frame shown in FIGS. 6A or 6B, as long as the interconnection is achieved by way of coaxial cables.
Further, there are customers who wish to continue operating the 2F-BLSR system with the construction of FIG. 7 for the optical telecommunication apparatus while opening the possibility of constructing the 4F-BLSR system in the future. In such a case, it is desired not to discard the existing optical telecommunication apparatus when constructing the 4F-BLSR system but to upgrade the existing optical telecommunication apparatus.
As to the first problem, it is well known that the complexity of wiring is successfully eliminated by using a printed circuit board. However, use of a printed circuit board in such a GHz system inevitably causes a problem of extensive electromagnetic emission. In order to avoid this problem, a shielding structure has to be devised in combination with the printed circuit board.
As to the second problem, it is necessary to form the optical telecommunication apparatus with a size such that two of the optical telecommunication apparatuses, each used for forming a 2F-BLSR system, are accommodated in the standard open-rack frame in order to form a 4F-BLSR system. In relation to this, it should be noted that each of the optical telecommunication apparatus has to have a shielding such that the node can be operated in the state equipped with only one of the optical telecommunication apparatuses while the half of the open-rack frame is empty.
Further, the optical telecommunication apparatus should be configured so as to allow an interconnection with another optical telecommunication apparatus to form a telecommunication apparatus for the 4F-BLSR system.
In addition, it is desired to provide an optical compensation device inside the open-rack frame in order to compensate for the dispersion of optical signals caused during the transmission through the optical fibers forming the optical link.